The present invention relates to optical method and apparatus for reading from and/or writing information on an information recording and/or reproducing medium such as an optical disk and, more particularly, to optical method and apparatus for detecting the focusing state and positional accuracy of a light beam directed onto an optical disk tracking guide in an optical read/write system.
In an optical system which reads out from or writes information on an information recording and/or reproducing medium, a laser beam is projected onto the information recording and/or reproducing medium (to be referred to as an optical disk hereinafter) through an optical head. The laser beam is reflected from the optical disk and is picked up by the optical head. In order to write information on the optical disk and read it out therefrom properly, the focal point of an objective lens of the optical head must be accurately placed on the light-reflecting surface of the optical disk. In other words, the beam waist of the laser beam converged by the objective lens must be projected onto the optical disk. Various apparatuses have been developed to detect the defocusing state or out-of-focus state of the objective lens. However, these apparatuses are unsatisfactory. For example, an apparatus which utilizes the difference between sizes of focused and defocused beam spots on the optical disk is proposed wherein different patterns of focused and defocused beam spots are projected onto a photodetector and are detected by the photodetector to control proper focusing. Further, a laser beam focusing apparatus which is provided with a lens system combining a convex lens and a cylindrical lens is disclosed in U.S. Pat. No. 4,079,247 of Bricot et al. In the above-mentioned apparatuses, if minute recesses or projections are formed on the optical disk, a diffraction pattern is formed in the beam spot pattern on the photodetector, resulting in erroneous operation. In an optical disk which has a tracking guide to increase information recording capacity, a diffraction pattern of the tracking guide is formed in the beam spot pattern on the photodetector when the beam spot is formed on the tracking guide. As a result, the apparatus may be erroneously operated.
In the apparatuses described above, the defocusing state of the objective lens is detected by changes in the size of the beam spot pattern on the photodetector or by the shape of the beam spot pattern. Another apparatus is proposed which detects the defocusing state of the objective lens by the location of a beam spot pattern formed on a photodetector. In this apparatus, a laser beam for detecting the defocusing state of the objective lens is incident on the objective lens and parallel to an optical axis of the objective lens and projected onto the optical disk through the objective lens. This laser beam is provided in addition to laser beams which are used for readout and writing of information. However, this apparatus has a drawback that the optical system becomes complex and the apparatus is manufactured at high cost, because an optical system having at least two optical paths for the laser beams is required. Other apparatuses are disclosed in Japanese Patent Disclosure No. 53-28405 and 53-10407, respectively. In these apparatuses, laser beams for reading out and writing information are not transmitted on an optical axis of an objective lens but are transmitted in parallel to the axis. However, in these apparatuses the laser beams cannot be sufficiently converged by the objective lens, and a sufficiently small beam spot cannot be formed on the optical disk. Further, since the laser beams pass through the outer peripheral portion of the objective lens and are projected onto the optical disk, the intensity of the laser beams projected onto the optical disk is decreased by eclipse.
Further, an apparatus is disclosed in Japanese Patent Disclosure No. 53-118103, in which a prism is arranged in an optical path of laser beams which have been reflected by an optical disk. In this apparatus, the intensity of the laser beams may attenuate when they pass through the prism, and unwanted diffraction may occur.
Although the various aforementioned drawbacks are peculiar to the optical system of an optical head, the inventor hereof has revealed that the photodetector provided in the system for detecting the defocusing state of the objective lens also involves a problem to be solved. In the conventional optical head, a diffraction pattern appears in the beam spot on the light receiving surface of the photodetector due to smaller projections or recesses on the light-reflecting surface and to dirt and defects on the optical disk and in the optical system. Due to the diffraction pattern, the photodetector judges the objective lens as being in the defocusing state in spite of the fact that the objective lens is in the in-focus of the just focusing state, there being a possibility that a focusing signal will be generated. As a result, it is not possible in the conventional optical head to correctly detect the just focusing state.